I have a 1991 Z28 305 with 63,000 miles. I have a problem with overheating. The car runs about 190º normally with the A/C off on flat surface. When I turn on the A/C the car runs about 220-230 º and when going uphill or when pushing the motor the car overheats but if I turn off the A/C the temperature slowly goes down to normal operating temperature around 200 º. I have replaces these items:

Thermostat - 190 º
Water pump
Radiator fans
Coolant
Flushed the engine
The motor only has no indication of water (coolant) in the motor oil
The Chevy dealer checked it and found no problem with the radiator.
Have new spark plugs and injectors

Any help is greatly appreciated.

Thanks

 

Dual fans right? The secondary fan should ALWAYS come on when the A/C is on regardless of any temperature - does it?

 

Its dual fans. One turns on when the temperature goes over 160 degrees and the other one turn when the A/C is turned on and stays on. I have a Hypertech fan switch that turns on at 160 degrees and I also have installed the Hypertech ThermoMaster Computer Chip (HYP-121342).
Thanks

 

Air dam on the car?

 

The Air dam is on the car.

 

Has the air conditioning system ever had anything done to it?

 

The A/C was converted from R12 to R134A this year. The A/C did not work before therefore I have no base for comparison.

 

I'll say replace the radiator. Lots of debris tends to collect between the radiator and condenser over the years, and it gets packed into the radiator fins.

 

Thanks for the help. I found out it that the timing was off. That helped.
Thanks

 

Wow, it's been a few years. Don't drive the car much, huh?

Did the timing fix the overheating or just help it?

 

I don't drive the car much. It was fixed almost a year ago. My 91 Z28 has 67900 miles.

 

Odd. I have this exact same issue with my car. The difference being that I'm still running the stock single fan setup (Yes the A/C commands the fan ON 24/7). I contribute my borderline overheating with A/C to the poorly designed single fan not pulling enough air through the radiator due to it both not covering enough surface area and not being sufficiently shrouded. At highway speeds, this isn't a huge problem (Around 200F with AC on).

When I bought my car back in 2008, it had no AC as it had been stripped by the ******* POs. Bought the parts and assembled the system myself. Charged with 134a. Blows nice and cold when the car is moving. It's idling through stop and go traffic that makes the engine temp shoot up and the AC performance go down the drain.

The one thing that I question however. Was GM really incompetent enough to let the car leave the factory with this design, or is it something else?

 

As I mentioned in a previous post, debris can collect between the radiator and condenser, restricting the flow. Even if cleaned out, there can still be a fair amount of blockage in the radiator's fins.

I've had my share of third gens with single electric fans and the AC worked fine at idle (in all temps) and it did not cause the engine to run hot.

 

The problem with that theory is that the radiator and condenser in my car are brand new.

 

It's not a theory, is a fact. It may not be your problem, but it does happen and cause the exact same symptoms. Your car does have an issue though, it's not just an inherent design flaw.

Your fan runs, but it's possible the motor is just plain weak and the fan doesn't move enough air. As the PO found out, what is your current timing advance set at? You could also have two separate problems. What are your AC pressures in hot weather when idling and just above idle? How much cooler does the car run when the AC is off? And are you saying your fan runs all the time, even with the AC off?

 

- Fan motor was recently replaced within the last few years and sounds strong.
- Initial advance is set to 8*BTDC and likes to hang around 16 - 20* at idle.
- I cannot remember the pressure readings of the AC system as it's been a few years. I calibrated it using a manifold set and typically get a 40F temperature delta between ambient and vent temps going down the highway.
- Idling temps in the hot summer are typically around 210 - 220F AC OFF and around 200F when the car is going down the highway AC ON. It is an airflow problem. The single stock fan has a very poor design.

 

IMHO, you are running a little too warm even with the AC off. My temps in all the cars have always been cooler, but I also generally run a 160 or 185 thermostat. I know that is like comparing apples to oranges, but the stock fan has always been able to keep the temps down around the correct temp of the thermostat. Ignoring the fan, I've never had one run anywhere near 200 going down the highway.

Are your temps based off of the stock gauge? If so, it could just be a false warm reading along with a compressor that is weak at idle. I've seen many many R4 compressors perform poorly at idle, right out of the box, whether new or reman. Just throwing some ideas out there.

And for the record, I do agree that the single fan is a poor design, but for most stock applications, it was a sufficient design.

 

Any updates?

 

My 84 had a stock single fan, stock radiator and upgraded to 134. I ensured all fins were strait and no debris between condenser and radiator. Driving on the highway, all was good. In town, no AC, 120* in New Mexico, no issues. However, as soon as I turned my AC on, the temps would climb unitil boiling over. Fan worked as designed and Air Dam is intact. On the highway, temps would start coming down.

Driving in the mountains near Riodoso, New Mexico (yes, they have mountains down there), AC on or off, the car's temp would climb and climb until boil over. I upgraded from stock radiator to BeCool aluminum radiator. Same exact issues.

Same with the mountains here in Utah. When it climbs to the red i had to pull over and let her cool off.



This past winter, I upgraded to a dual TPI fan set up. I'm interested to see if it makes any difference or if I need to upgrade to an LS1 style set up. I hope not because I have no fabrication experience to make brackets.

 

Try taking it out of OD,hills put a big strain on the engine and tranny.

 

I had the same problem on my 88 IROC running a 383 TPI with dual stock fans in hot, humid Florida and I have done all of the things you described. I also run a BeCool radiator and still had an overheating problem but I did finally solve it and I think this may work for you as well assuming that everything in your cooling system is functioning properly. This will be a long recommendation but give it a try.

Coolant velocity is very important because it affects the convective heat transfer that needs to take place. When your coolant temperature is at or above your thermostat temperature then your thermostat should be fully open and is nothing more than a flow restrictor and you are still overheating.

Try this simple experiment. Temporarily remove the thermostat and then go drive the car however you want with the AC on and see if it will overheat. In this situation, you are at your maximum coolant velocity with your current water pump. If the car does not overheat then you have proven that more coolant velocity will solve your problem. I would expect that you will find the car is running with too low of a coolant temperature in this condition. That is what happened to me when I ran my car without the thermostat; I had difficulty getting the coolant temperature to raise much at all which proved that increased velocity worked. Beware of the accuracy of the stock water temperature gage. If the car still overheats with the thermostat out then look for flow blockage and consider installing a high flow water pump.

If you experience the same result that I did then you will have proven that you also need an increase in coolant velocity to solve your problem. Furthermore, you will have proven that you have too much of a flow restriction (Low Velocity) with your thermostat and not enough of a flow restriction (Too High Velocity) without a thermostat because the engine will run too cold, i.e. coolant temperture below 185 deg F. This will tell you that the answer is "somewhere in the middle" and you can work with that information.

I do not recommend driving the car long term without a thermostat because you still need to manage your cooling system's temperature. Also, the coolant temperature sensor is an important input to the computer so you may see your mileage temperarily decrease because the computer will think you have a "cold engine" if you leave the thermostat out long term.

At this point, you need to put a thermostat back in the car. Go to your favorite auto parts store and buy a good quality high flow thermostat but avoid the ones that "lock open" and buy a few gaskets. If you look closely at the high flow thermostat then you should see that the hole diameter that the thermostatically controlled plug seals against is bigger than a normal thermostat. My experience is that this helps but is still not enough but you should give it a try since our engines are different.

The next step is to carefully drill some holes around the circumference of the thermostat. These holes will obviously always be open so you are now adding additional "flow area" to supplement the amount of flow area (i.e. hole diameter) that is available as the thermostat opens. You can obviously add holes but you can't take them back. So at this point, we have to do some "trial and error" and drill a few holes, reinstall the thermostat, and then go road test the car to see if it is over heating. If it is overheating, then pull the thermostat and add some more holes and try again. Remember, the initial experiment proves the answer is "in the middle" so go slow and progressively add holes to the thermostat until you find that flow area combination that works. I would recommend targeting a coolant temperature around 185 deg F.

It took me three attempts at drilling holes but I finally solved my overheating problem. I used a cheap infrared thermometer and kept recording my coolant temperature versus hole combination until I got the result I was seeking and it worked. So, I was able to solve my problem for less than $20. Final comment, I also run a high flow water pump and you may want to do that as well. Mine is just an auto part store pump, nothing special.

Hope this works for you. Good luck.

 

That's a good point. Drilling holes has been a time honored tradition for extra cooling flow but honestly, wasn't something I thought of in this case. Good call!

 

MR Gasket makes a high-flow thermostat that's 10 or 12 bucks. A lot of parts stores have them in their "performance" isle now. It uses a very large opening, shouldn't need to drill holes at all.